Although the gene for Huntington's disease (HD) has been identified, the processes that lead to cell pathology and degeneration are still unclear. The proposed studies examine early pathology and pathophysiology in two transgenic mouse models of Huntington's disease. Our preliminary data in one mutant model, the reversible HD94 model, suggest that the chemoarchitecture of the striatum is altered in early symptomatic stages of the disease: there are more mu opioid receptor-rich striosomes in mutants than in controls, which may lead to an imbalance of activity between the striosome and matrix compartments and produce the symptoms of chorea and involuntary activity. The studies will determine whether there are more striosomes in mutants by using immunocytochemical methods to identify striosomes. The working hypothesis states that the early stages of the disease are associated with abnormal developmental processes. The specific hypothesis is that a critical part of the pathology underlying the symptoms and final degenerative process of Huntington's disease is abnormal neurogenesis prenatally and postnatally, and that the number of striosomes in mutants reflects neurogenesis abnormalities. In addition, our model of the behavioral functions of striosomes predicts that mutants will be more sensitive to dopamine agonists. Finally, the studies will investigate prenatal and adult cell proliferation and neurogenesis in mutants and their controls by using a thymidine analogue, bromodeoxyuridine (BrdU). The studies may provide clues to the function of the gene huntingtin, and define a target for therapeutic strategies. In addition, these studies address the plastic and proliferative capacity of the adult brain.